Cooking liquor for the production of sulphite pulp



Patented Sept. 1, 1931 UNITED STATES PATENT OFFICE GEORGE RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY,

OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE COOKING LIQUOR FOR THE PRODUCTION OF SULPHITE PULP 1i Drawing.

This invention relates to the production of chemical wood fiber by the sulphite process of digestion, and has for its object the production of ahigh grade sulphite pulp which 5 is suitable for manufacture into bond, ledger and other writing paper of the highest quality, and which is an excellent raw material for the preparation of fiber of high alpha or resistant cellulose content.

For the production of sulphite pulp of high quality, it has been found preferable to digest wood chips in a liquor containing a soluble sodium salt, as, for example, in a sodium bisulphite solution having a combined S0 content of about 2%, orgreater, and a freeSO content of about 5%. Such a liquor is particularly effective in penetrating the chips and in reacting with non-cellulosic constituents thereof, yielding an easily bleachable pulp having excellent papermaking characteristics. The use of a sodium bisulphite solution as the digestin li nor, however, entails certain distinct dlsa vantages. Thus, in actual practice I have found that such liquor tends to wear away the brick and cement lining of the usual sulphite digester, and that such lining is impaired far more rapidly and severely than when the usual calcium bisulphite liquor containing about 1% combined and 5% free S0 and employed for the production of the usual commercial sulphite pulp, is employed. This disadvantage incident to the use of a solution of so dium bisulphite may be ascribed to the fact that, whereas a calcium bisulphite liquor assists in the preservation of the lining by depositing insoluble calcium salts, such as calcium sulphate and sulphite, in the form of a hard-baked scale on the lining during digestion, there is no tendenc for such scale formation from a sodium bisulphite solution, because of the high solubility of the corresponding sodium salts. Consequently, instead of protecting the lining, a sodium bisulphite solution doubtless reacts with and dissolves the lining material during digestion. Moreover, in a plant which, due to financial or other reasons, is not equipped with .a system for the recovery of the sodium constituent in the spent liquor resulting from Application filed November 1, 1926. Serial No. 145,708.

sulphite liquor is far more expensive than I corresponding calcium base material.

As previously indicated, in order to pro-- duce a sulphite pulp possessing optimum papermaking qualities, the liquor should contain considerably in excess of 1% combined S0 Because of the limited solubility of calcium bisulphite, however, if under the conditions of temperature and pressure maintained during digestion, a calcium bisulphite liquor is employed having a. combined S0 content considerably in excess of about 1% (which is the approximate limiting value of its solubility under digester conditions) difficulty is encountered because of the heavy precipitate of insoluble'calcium salts which is present in the liquor. Thesesalts become fixed on the pulp, and because they are exceedingly diflicult to remove by washing, they impair the commercial value of such pulp.

I have found that if a bisulphite liquor is employed, containing sufficient bisulphite or other salt of the alkaline earth metal group to precipitate an insoluble salt when S0 is released therefrom, and containing suflicient sodium bisulphite to produce a combined SO content to ensure rapid penetration into the chip and the removal of non-cellulosic constituent, an easily bleachable pulp having excellent paper-making characteristics may be produced without encountering the disadvantages incident to the use of a solution of sodium bisulphite only. Thus, digestion in.a sodium bisulphite liquor containing in solution suflicient alkaline earth metal bisulphite to precipitate alkaline earth metal monosulphite during the relief of S0 from the digester, results in the preservation of the digester lining. This is due to the deposition of insoluble alkaline earth metal salt in the form of scale on the linin Where a sodium iisulphite liquor containing calcium bisulphite is employed, the percentage of combined SO maintained by the calcium bisulphite may amount to 1% to 1.3%, depending on whether dolomite or calcite limestone, or lime, is employed as the raw material, and the remaining desired combined S0 content is afforded by the sodium bisul phite. Because of the high solubility of magnesium bisulphite, by using dolomite limestone as a raw material for supplying the calcium constituent, the percentage of combined SO which may be maintained in solution other than furnished by sodium bisulphite may amount to a somewhat higher percentage than that maintained with a liquor prepared from either calcite limestone or lime, or both.

In actual practice, the liquor contains approximately 1% combined S0 which is maintained by the calcium bisulphite, and the combined SO content in excess of 1% is maintained by the sodium bisulphite. The liquor also contains the desired amount of free S0 to effect the desired fiber liberation, this SO content amounting to about 5%, more or less. Thus, a typical composite cooking liquor for the production of a high grade sulphite pulp should contain about 1% in the form of calcium bisulphite, about 1% combined SO in the form of sodium bisulphite, and about 5% free S0 A great advantage obtained by the use of calcium compounds 1s the economy of operation which may be realized, since such calcium compounds are relatively much cheaper than sodium base compounds such as sodium carbonate or caustic soda. This economy is particularly true in a plant which is operated without a system for salvaging or recovering the inorganic compounds contained in the spent liquor resulting from digestion. The substitution of calcium for a fraction (say half or more) of the sodium constituent results in a very material saving in the cost of production, without, however, sacrificing the quality of the product. As a matter of fact, such fiber when tested indicates that it is as good in quality as the fiber produced by the digestion of chips in a 52 liquor containing only sodium bisulphite.

Whereas the use of calcium bisulphite together with sodium bisulphite is advantageous and preferable in a plant wherein no recovery system is employed, considerable difficulty is experienced if the recovery of the inorganic constituents of the spent liquor is attempted. This is due to the fact that the concentration of the spent liquor to remove a portion of its aqueous content prior to smelting produces a heavy precipitation of insoluble calcium salts and the formation of a thick scale in the preheaters and evaporators. Subsequent smelting results in formation of calcium sulphate and calcium carbonate,

which produce a heavy precipitate when the smelt is dissolved in water, thus necessitating filtration of the smelt solution. Consequently, when calcium bisulphite is employed together with sodium bisulphite, it is impractical and uneconomical to practise recovery of the sodium constituent.

When such a process of digestion as hereinbefore described is practised in a plant equipped with a recovery system, a sodium bisulphite solution containing, say, about 2% combined S0 and containing suitable barium or strontium salts is substituted for one containing calcium bisulphite. Such substitution permits a recovery of the sodium constituent of the spent liquor. For example, barium salts e. g. barium bisulphite, are used (or produced in the sodium bisulphite solution) insuflicient proportion so that when the digester is being relieved near the end of the cook, with the escape of free S0 the insoluble barium monosulphite is precipitated. Bariumsulphite and barium sulphate, however, are far less soluble than the corresponding calcium salts, so that only a very small amount of barium bearing material need be added to the digesting liquor to result in the precipitation of such salts during the final period of digestion. Thus about '10 to 30 pounds of barium carbonate need be added to the usual digester charge, the barium carbonate reacting with the sulphurous acid and forming barium bisulphite, whichtoward the end of digestion is gradually precipitated as barium sulphite and sulphate.

The precipitation of barium monosulphite together with some sulphate gradually takes place as the free sulphurous acid concentration of the cooking liquor is gradually decreased by the relief of free S0 from the digester toward the end of the cook. Initially the liquor may be clear, as the high concentration of sulphurous acid increases the solubility of the monosulphite by reacting therewith to form the bisulphite which is more soluble than the monosulphite, but as the sulphurous acid which maintains the monosulphite in solution as bisulphite is decreased in concentration, a precipitation of the more insoluble monosulphite takes place. For example, if the usual clear calcium bisulphite cooking liquor is boiled and the free S0 liberated permitted to escape, a heavyprecipitate of calcium monosulphite results. So, too, when a clear cooking liquor containing barium bisulphite is employed in a digester, as the liberation and escape of free S0 proceeds, the more insoluble barium monosulphite is precipitated and deposits as scale on the digester lining.

The spent liquor resulting from digestion contains, however, but a small proportion of barium salts, so that it may be subjected to the evaporation and concentration steps without encountering excessive difliculty due to scale sulphite.

formation. Only a slight precipitate of barium sulphate and carbonate is produced in the smelt solution, so that filtration thereof is unnecessary. The smelt liquor may then be treated to produce a fresh sulphite liquor, sufiicient bariumand sodium-bearing constituent being added to compensate for any losses in the cycle.

The barium-bearing material may be barium carbonate, barium hydroxide, or barium sulphate. Barium sulphate remains as such in the digesting liquor, only a relatively small proportion thereof being converted to barium bisulphite.

In lieu of barium bisulphite, strontium bisulphite may be used or formed in the sodium bisulphite solution in suflicient proportion to ensure toward the end of the cook the precipitation of strontium sulphite and sulphate which form as scale on the digester lining. Strontium sulphite is even more insoluble than barium sulphite so that in many cases it may be preferable to employ a sodium bisulphite solution containing strontium sulphite as the cooking liquor. Strontium sulphate, however, is more soluble than barium sulphate, so that if only sulphate is the salt used in the cooking liquor, it is preferable to employ barium sulphate.

I am aware of the fact that composite base liquors have been employed for the production of sulphite pulp, but the pulp produced therewith is of the usual commercial variety. For example, I am aware that it has been proposed to use certain small amounts of sodium compounds together with calcium bi- As emphasized hereinbefore, the liquor herein employed has a combined S0 content materially in excess of the combined SO content of the usual calcium bisulphite liquor and contains sufiicient insoluble alkaline earth salt, and more particularly either calcium bisulphite or barium or strontium bisulphite to ensure precipitation during digestion. A sodium bisulphitesolution containing calcium bisulphite is preferred when a plant is not'e'quipped with a recovery system, and one containing barium bisulphite or strontium bisulphite. is preferred when a plant is equipped with a recovery system.

\Vhile I have described the use of a cooking liquor consisting of a sodium bisulphite solution containing salts of the alkaline earth metal group, it is alsoadvantageous to emplo such alkaline earth metal salts in acid solutions containing other soluble salts and employed as cooking liquors for the production of high grade suphite pulp. Thus a cooking liquor consisting of a sulphurous acid solution of sodium sulphate is disclosed in my U. S. Patent #1,427 ,125 issued Aug. 29, 1922, as being adapted for the production of a high grade sulphite pulp. Where such a liquor is employed, the pulping or fiber-liberating action is believed to be persodium sulphate. Such a liquor contains a relatively high concentration of sodium constituent and is preferably employed in a plant equipped with a recovery system. Hence it is preferable to saturate substantially such a liquor with insoluble barium or strontium salts, which are more highly insoluble than the corresponding calcium salts. Thus, if basic barium salts, e. g. barium carbonate, is added to the liquor, barium sulphate is precipitated. If 'strontium carbonate is added thereto, strontium sulphite (which is more insoluble than strontium sulphate) is precipitated. Owing to the fact that barium sulphate has a much lower solubility than the other salts, it is preferably produced in the liquor.

By the expression substantially saturating the solution,'I means producing a sufiicient concentration of compounds in the solution to produce an initially slight precipitation or turbidity therein, or to produce a slight precipitation or turbidity'during digestion. and relief of S0 By the term soluble salt or soluble bisulphite as employed herein, I vmean a highly soluble salt or bisulphite, such as sodium sulphate and sodium bisulphite, as distinguished from barium bisulphite and a highly insoluble salt such as barium sulphate.

So far as generic subject matter is concerned, this is a continuation of my application, Serial No. 94,621, filed March 13, 1926.

What I claim is: g

1. A composite base acid sulphite liquor having a combined S0 content in the form of alkaline earth metal bisulphite and alkali metal bisulphite, the alkaline earth metal bisulphite being present substantially to the limit of its solubility and at least 1% combined S0 being in the form of alkali metal have aflixed GEORGE A. RICHTER. 

